90 research outputs found

    Seasonality in future tropical lower stratospheric temperature trends

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    The seasonality of the 21st century trends in tropical lower stratospheric temperature (LST) is examined in simulations by a group of comprehensive chemistry-climate models. In contrast to the past LST trends, there is robust seasonal dependence among ensembles of the same model. Furthermore, most models show strongest cooling around July–September and minimal cooling in February–March, which results in a weakening of the seasonality in tropical LST. Sensitivity simulations with isolated forcing reveal that greenhouse gas increases dominate the future tropical LST trend. This seasonally varying LST trend is linked to changes in the Brewer-Dobson circulation (BDC). The BDC can influence the LST through direct dynamical heating/cooling and indirect radiative effects primarily from ozone changes due to vertical transport. The latter is found to be the main cause for the seasonality of the 21st century LST trend, while it is difficult to separate them in the past

    Upward Wave Activity Flux as a Precursor to Extreme Stratospheric Events and Subsequent Anomalous Surface Weather Regimes

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    It has recently been shown that extreme stratospheric events (ESEs) are followed by surface weather anomalies (for up to 60 days), suggesting that stratospheric variability might be used to extend weather prediction beyond current time scales. In this paper, attention is drawn away from the stratosphere to demonstrate that the originating point of ESEs is located in the troposphere. First, it is shown that anomalously strong eddy heat fluxes at 100 hPa nearly always precede weak vortex events, and conversely, anomalously weak eddy heat fluxes precede strong vortex events, consistent with wave–mean flow interaction theory. This finding clarifies the dynamical nature of ESEs and suggests that a major source of stratospheric variability (and thus predictability) is located in the troposphere below and not in the stratosphere itself. Second, it is shown that the daily time series of eddy heat flux found at 100 hPa and integrated over the prior 40 days, exhibit a remarkably high anticorrelation (−0.8) with the Arctic Oscillation (AO) index at 10 hPa. Following Baldwin and Dunkerton, it is then demonstrated that events with anomalously strong (weak) integrated eddy heat fluxes at 100 hPa are followed by anomalously large (small) surface values of the AO index up to 60 days following each event. This suggests that the stratosphere is unlikely to be the dominant source of the anomalous surface weather regimes discussed in Thompson et al

    Overview of Large-scale Tropospheric Transport in the Chemistry Climate Model Initiative (CCMI) Simulations

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    The transport of chemicals is a major uncertainty in the modeling of tropospheric composition. Here we compare the large-scale tropospheric transport properties among different models in the Chemistry Climate Modeling Initiative (CCMI) with a focus on transport defined with respect to the Northern Hemisphere (NH) midlatitude surface. Among simulations of the recent past (1980-2010) we show that there are substantial differences in their global-scale tropospheric transport properties. For example, the mean transit time since southern hemisphere air last contacted the NH midlatitude surface differs by ~30-40% among simulations. We show that these differences are most likely associated with differences in parameterized convection over the oceans, such that the spread in transport among simulations constrained with analysis fields is as large as the spread among free-running simulations

    Large-Scale Transport Responses to Tropospheric Circulation Changes Using GEOS-5

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    The mean age since air was last at the Northern Hemisphere midlatitude surface is a fundamental property of tropospheric transport. Recent comparisons among chemistry climate models, however, reveal that there are large differences in the mean age among models and that these differences are most likely related to differences in tropical (parameterized) convection. Here we use aquaplanet simulations of the Goddard Earth Observing System Model Version 5 (GEOS-5) to explore the sensitivity of the mean age to changes in the tropical circulation. Tropical circulation changes are forced by prescribed localized off-equatorial warm sea surface temperature anomalies that (qualitatively) reproduce the convection and circulation differences among the comprehensive models. Idealized chemical species subject to prescribed OH loss are also integrated in parallel in order to illustrate the impact of tropical transport changes on interhemispheric constituent transport
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